Adding warning for RethinkDB security issue on public installations

Currently, the shipyard quick start creates an installation with public ports exposure. If someone has a Docker installation directly accessible from Internet, without firewall closing ports, anyone could so access to RethinkDB, create an account and have full control on Docker instances (and from there, to the host server).
Could you update this to say "your RethinkDB installation may publicly listen".  Just because the machine is on the public internet does not necessarily mean the ports are open to the world.

I agree on the warning though.